High Level Waste

High Level Waste (HLW) and Spent Fuel

High level waste is radioactive waste with a high level of radioactivity that exceeds limits set by the authorities and radioactive substances in appreciable concentrations that produce large quantities of heat. The majority of this waste is spent fuel from nuclear power plants.

Spent Fuel Characteristics

Nuclear fuel consists of a series of cylindrical ceramic pellets of uranium oxide, whose U-238 is enriched to a variable degree (up to 5%) with U-235. These are encased in zirconium alloy tubes to create fuel rods, which are arranged into a structure to form the fuel assembly.

Since it is in the reactor, the uranium and other radionuclides generated are subject to neutron capture and nuclear fission reactions, resulting in fission products, activation products and the generation of plutonium and minor actinides. The composition of these products includes virtually every element of the periodic table.

Quantities and characteristics of the components of the irradiated fuel depend on its initial U-235 enrichment, its degree of burnup and the operation of the reactor.

infographic of a fuel element
Fuel assembly model

The Nuclear Fuel Cycle

The activities involved in the nuclear fuel cycle span from the extraction of uranium ore to the final management of spent fuel, through intermediate stages such as the enrichment of the ore, the manufacture of fuel assemblies and their use in nuclear power plants.

The spent fuel unloaded from the reactors of nuclear power plants can be managed in various ways. Each of them is known as a fuel cycle.

Explanatory diagram of open and closed spent fuel cycles

Open Fuel Cycle

In an open or once-through fuel cycle, the spent fuel is classified as high level radioactive waste and is managed as such in specific interim storage facilities.

The final placement of the spent fuel will be a deep geological repository. It entails its final disposal as waste.

Closed Fuel Cycle

In the closed fuel cycle, spent fuel is partially reused by means of reprocessing and recycling. It consists of retrieving the components in the spent fuel which have energy potential, primarily uranium and plutonium, for being reused in nuclear reactors. The resulting fuel, consisting of plutonium and uranium oxides, is called MOX (mixed oxide) fuel and can be further reprocessed.

The remaining components of the spent fuel (fission and activation products and other actinides and structural materials) are classified as waste, and are conditioned and transported to a storage facility. Final management, as in the open fuel cycle, entails disposal in a deep geological repository.

Interim Storage Systems


Pool Storage

Once the spent fuel is unloaded from the reactor, it is temporarily stored in pools and is located within the nuclear power plant, for cooling.

Water is used for immediate storage due to its high heat transfer coefficient, its good shielding properties, its transparency and its manageability.

Cask Storage

Sometimes, the pools reach capacity or it is necessary to remove the fuel in order to begin dismantling the plant. When this occurs, the fuel is loaded into casks that are stored in a dedicated onsite installation, known as an Individual Interim Storage Facility. The casks used in Spain for interim storage are made of metal or concrete and metal.

The Individual Interim Storage Facility at the Trillo Nuclear Power Plant

The Trillo nuclear power plant has used a cask storage system to house the facility’s spent fuel since 2002 on a temporarily basis. The installation has concrete walls and roof, and it is able to accommodate up to 80 dual-purpose casks (for storage and transportation).

The Individual Interim Storage Facility at the José Cabrera Nuclear Power Plant

This facility has been constructed on the site of the plant and has been designed for the dry storage of all spent fuel unloaded from the reactor. Its construction was essential to the dismantling of the plant.

It consists of a slab of reinforced concrete supporting 16 storage modules, enclosed by a simple exterior fence system for radiological protection (the area beyond this fence is classified as a free access area) and an interior physical security fence delimiting the storage area.

This Interim Storage Facility houses 12 modules loaded with spent fuel and 4 extra additional casks containing the most highly radioactive metal pieces from the segmentation of the reactor internals.

The Individual Interim Storage Facility at the Ascó Nuclear Power Plant

This installation has been constructed on the site of the plant designed for the dry storage of the spent fuel unloaded from the two reactors.

It consists of two 16-storage-module slabs of reinforced concrete with capacity for 16 storage modules per slab. It is enclosed by a simple exterior fence system for radiological protection (the area beyond this fence is classified as a free access area) and an interior physical security fence delimiting the storage area.

The CTS Project and its Licensing Process

On 30 December 2011, the Council of Ministers designated the municipality of Villar de Cañas in Cuenca as the site for the Centralised Storage (CTS) facility and its Associated Technology Centre.

This facility will provide temporary storage for all spent fuel and high level waste from Spanish nuclear power plants. The decision to store these materials in a single location was based on a December 2004 Resolution by the Congressional Commission on Industry, which urged the Government, in partnership with Enresa, to develop criteria for establishing a centralised storage facility for spent fuel storage and high level radioactive waste in Spain.

An Interministerial Commission, created in 2006 and composed of representatives of the principal ministries, was assigned the task of establishing the criteria to be met by the site for the CTS and its Associated Technology Centre.

Location of Villar de Cañas (Cuenca)

In June 2006 the 6th General Radioactive Waste Plan is approved, and sets the CTS as a priority.

From the beginning of 2012, Enresa has been working on the characterisation of the site for the CTS and the detailed design of the facility. In January 2014, applications for the prior (siting) authorisation and the construction permit were submitted to the Ministry of Industry, Energy and Tourism (MINETUR). These permits, granted by the MINETUR subsequent to approval by the Nuclear Safety Council, issued on 28 July 2015, and the Environmental Impact Statement from the Ministry of Agriculture, Food and the Environment (MAGRAMA), are required, along with the mandatory construction permit issued by Villar de Cañas Council, before construction can begin on the CTS.

Once the construction of the CTS is complete, it will require an operating permit issued by the MINETUR, which must be accompanied by further authorisation from the autonomous communities and the EU (EURATOM).

The CTS has been designed for a 100-year life, although the current General Radioactive Waste Plan (GRWP) sets out an operational life of 60 years. After this time, the radioactive material will be removed for subsequent management and the facility will be dismantled, as with any other nuclear facility at the end of its operational life.

International Experience: HABOG

There is extensive global experience in the operation of this type of facility. The technology employed in each interim storage system varies from country to country, some using pool storage and others dry storage (casks, vaults, etc.). Dry storage technology using vaults, as used in Spain, has been licensed and implemented for several years in other countries such as the United States, France, United Kingdom, Hungary, and the Netherlands.

The Dutch CTS, Habog, became operational in 2003. It is located in the industrial area of Vlissingen-Oost, in the south west of the Netherlands, alongside companies from other sectors. This facility, which uses the system to be implemented in Spain, was designed to store the country’s spent fuel and vitrified waste for 100 years.

Image of HABOG, The Dutch CTS

The Spanish CTS

The CTS is an industrial facility designed to accommodate the spent fuel from Spanish nuclear power plants and other high level waste in a single location. According to the 6th GRWP, currently in force, there is an estimated 12,000 m3 of materials requiring management, most of them are spent fuel (about 20,000 fuel assemblies).There are only small quantities of vitrified waste (less than 70 canisters) and special waste.

The CTS will be a dry storage surface facility, ensuring the confinement of these materials by means of a multiple-barrier system. This passive system facility, with a modular and reversible design, will allow the removal of the spent fuel and high level waste for subsequent management once the operational life of the facility is at an end. In this way, it will enable separation of the different stages of management.

The CTS will centralise all necessary processes for the interim management of all the spent fuel that, until now, has remained at the nuclear power plants, either in pools or in supplementary dry cask storage systems. The centralisation of these materials facilitates safety and surveillance. It also enables optimisation of the resources required for their monitoring.

The integral management of spent fuel and high level waste will facilitate the dismantling of Spanish nuclear power plants as they reach the end of their operational life. It will also enable the return and storage of the high level waste produced in the dismantling of the Vandellós I Nuclear Power Plant, temporarily stored in France.

Parts of the CTS and its Primary Process

The area designated for the construction of the CTS in Villar de Cañas (Cuenca) has a surface area of 53 hectares, in which the following facilities will be located:

Nuclear Facility (CTS)

The buildings and installations that make up the nuclear facility fall into two groups, differentiating between those which are located inside the protected area (within the double security fence), which are subject to the requirements of the Regulation on Nuclear and Radioactive Facilities, and those which are located outside the protected area (beyond the double security fence), which are not subject to this regulation.

CTS Primary Process

The primary process of the CTS will be undertaken in a row of buildings incorporating the reception building, the process building and the storage vaults. The diagram of the nuclear facility’s general layout shows the following buildings and structures:

Reception Building

Here transport vehicles carrying casks of spent fuel, special waste, and canisters of vitrified waste are received.

Process Building

Here the various mechanical processes for preparation of the casks are undertaken (checking, removal of covers), in two separate lines for spent fuel and canisters of vitrified waste. This building will also be used for unloading the casks, temporary storage of fuel assemblies and encapsulation of spent fuel in the hot cell, as well as its removal.

Storage Vaults

A group of twelve storage vaults will be built in three phases - Phase 1 (Vaults 1-4), Phase 2 (Vaults 5-8) and Phase 3 (Vaults 9-12). These twelve vaults will enable the storage of spent fuel and canisters of vitrified waste in a total of one thousand four hundred and forty (1,440) storage pits or tubes. These will be arranged in a 10 x 12 formation in each vault.

The storage tubes will be made of stainless steel and the canisters containing radioactive waste will be deposited inside them. It will form a double confinement barrier, with the canister itself comprising the first barrier and the storage tube comprising the second one. The vaults are structures with extremely thick concrete walls that act as shielding, with independent air inlets and outlets to allow natural convection to cool the waste. Air circulates between the storage tubes and allows the natural removal of residual heat from the stored materials, without entering into contact with them at any time.

A ventilation shaft reaching a height of 45.5 m will be constructed above ground level and over each vault (one per vault), seated on a reinforced concrete box girder.

Special Waste Storage Module

Which is composed of four structurally independent storage units and a number of adjoining auxiliary structures, assigned to the storage of what is termed special waste:

  • Pit storage unit
  • Source storage unit
  • Operational waste storage unit
  • Reserve storage unit
Cask Holding Facility

Its main function is to temporarily house transport and/or store casks for spent fuel and canisters of vitrified and special waste prior to their delivery to the process area. The existence of this holding facility will allow the main facility to absorb higher inflows of waste streams, especially during the first years of the facility’s operation, when a build-up of demand is expected. It has capacity for 80 loaded casks.

Cask Maintenance Workshop

Its main function is the maintenance, both external and internal, of transport casks.

Laboratory for Spent Fuel and Radioactive Waste

Its belongs to the Associated Technology Centre, but due to its nuclear-related functions it will be located inside the protected area (within the double security fence).

Support Facilities

Inside the protected area, the nuclear facility will be equipped with the following support facilities:

Auxiliary services building

It will house the areas and equipment for auxiliary functions (including staff access, ventilation equipment, empty canister storage, etc.).

General services building

It will house some of the process, control and management support services, such as the medical service, changing rooms, laundry, etc.

Technical services building

It will centralise the majority of the facility’s services, including hot water, purification, production of demineralised water, fire protection, etc.

Access control and physical safety building

For physical protection functions and control of access to the protected area.

The radioactive waste treatment building for waste produced in the operation of the CTS, Enresa's offices, the empty cask storage area, and the electricity building will be also included inside the protected area.

Facilities outside the Protected Area

Other facilities will be located outside the protected area: the service area building; the workshops and services building; the auxiliary building; the vehicle shed; the meteorological, seismic and environmental data centre; the waste water treatment plant; the stormwater ponds; the electrical substation and the processing centre.

Associated Technology Centre

The CTS’s Associated Technology Centre will be a supplementary installation for the infrastructures developed through Enresa’s R&D programmes over the last 30 years. It will also provide support to the CTS during both its construction and its operation phases.

Its activities will be developed within specific projects and programmes and will be undertaken in four primary laboratories, one of them, the Laboratory for Spent Fuel and Waste, will be located within the nuclear facility. The other three conventional laboratories will be located outside the nuclear facility:

Environmental and Process
Characterisation Laboratory

This laboratory will have its own analytical resources for conventional (non-radioactive) materials and will develop specific tests related to the chemical behaviour of confinement systems and materials, as well as to environmental dispersion and migration processes.

Technologies and methodologies related to environmental surveillance and monitoring at all levels will be given a special focus, as well as the characterisation of coupled processes arising from the use of materials in the design of confinement systems, where their applicability and durability are closely associated with their chemical behaviour.


The purpose of this laboratory will be to conduct research and characterisation testing related to various metallic materials, cement and clay, used as common components in barriers for containment and confinement systems in radioactive waste storage facilities.

The materials laboratory will draw together all the developments made to date, undertake complementary activities for selecting confinement materials and provide support for the construction of the storage facilities.

Prototypes Laboratory

The purpose of this laboratory is to verify, at a scale of an industrial prototype system, the technological advances produced by R&D which have been successful on a small–scale system and are of crucial application in activities or projects for radioactive waste management.

Prototypes to be tested will be related to the operation of the CTS, the dismantling of nuclear facilities and the verification of components and operating systems for long-term geological storage and disposal.

The project will include the necessary infrastructure for the location and establishment of businesses, which will provide support to the CTS and the Associated Technology Centre.

There are plans for the development of a future business park, and a Business building for companies. There will be a laboratory and an auxiliary warehouse for using in the construction of the CTS.

There are also plans for the Business Park to have an industrial estate together with a technology zone. Activities necessary for the park’s land development and general service infrastructures are also planned.

Transport of High Level Waste (HLW)

Spain does not currently transport any high level radioactive waste or spent fuel. These materials remain in the power plants’ onsite pools or individual interim storage facilities.

Prior transport activities

A Transport Plan was drawn up prior to the creation of the Spanish Centralised Storage facility, to enable high level waste and spent nuclear fuel to be transported from nuclear power plants to this new facility. In order to undertake this type of transport operation, regulations and criteria already used for the transport of low and intermediate level waste will be applied.

Packaging will be used for the transport of high level waste and spent nuclear fuel, which is designed to withstand the accident conditions defined by the ADR (European Agreement concerning the International Carriage of Dangerous Goods by Road).

In global terms, the transportation of these materials has covered more than 30 million kilometres and there has never been a radiological incident.

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